Патент USA US3031347код для вставки
United States ice we 3,031,333 Patented Apr. 24, 1962 2 solution of chromic acid and a reducing agent which is 3 031 333 _ CORROSION REsIsrA’Nr ’COATED STEEL MEM BERS AND METHOD OF MAKING SAME _ compatible with the chromic acid. This compatibility means that the chromic acid and the reducing agent, even though both are present in the solution, will not in the appropriate dilution react rapidly with each other and will not form a visible precipitate while the solution is being to Pennsalt Chemicals Corporation, Philadelphia, Pa., contacted with the steel. a corporation of Pennsylvania The steel sheet wet with the solution is then dried at N0 Drawing. Filed June 20, 1956, Ser. No. 592,552 a temperature above 212 degrees F. During this drying, 2 Claims. (Cl. 117-50) 10 the chromic acid reacts with the reducing agent and This invention relates to coated steel members, more becomes partially reduced. As va result, the ?nal coating particularly to such members which are protected from is a combination of hydrated chromium oxides containing corrosion. , about 20 to 60% chromium by weight. Among the objects of the present invention is the provi An inordinately high degree of corrosion resistance is sion of novel coated steel members which are simple and 15 obtained when 40 to 95% by weight of the chromium in inexpensive to make and yet are extremely resistant to the ?nal coating is in What corresponds to- the trivalent corrosion. condition, the remainder being hexavalent, and the coat Additional objects of the present invention include the ing itself weighs about 10 to 200 milligrams for every provision of a novel method for making such members. square foot of surface that it covers. Coatings of less Ludwig K. Schuster, Philadelphia, and Alfonso L. Baldr, Jr., Drexel Hill, Pa., assignors, by mesne assignments, . The above as well as still further objects of the present invention will be more clearly understood from the following description of several of its exempli?cations. Plain carbon steels such as ordinary SAE 1010 steel, weight give less protection, although coating weights need not be more than about 30 milligrams per square foot to the canning industry, are relatively inexpensive and give all the protection that is needed. If the reducing agent is omitted from the coating bath, dried coatings of the above weight range will not show the desired trivalent chromium content. They also tend simple to fabricate and use in practically any desired con to be somewhat deliquescent and therefore inferior. or the conventional Blackplate sheets that are used in ?guration. Unfortunately, these steels =are extremely - Typical reducing agents suitable for use in connection with the present invention are organic polyalcohols such as sugars, including invert sugar, sucrose, dextrose, glycol vulnerable to corrosion. In fact, unless the steel is pro tected as by a ?lm of oil, it will usually corrode even before it can be delivered by the manufacturer to the 30 and polyethylene glycols, glycerine, mannitol, sorbitol, consumer. triethanolamine, hydroxylamine salts such as its sulfate However, by reason of the low cost and extreme versa and hydrochloride, phosphorous acid, and potassium tility of these steels, many different types of treatment iodide. have been developed to reduce their corrosion so that Care should be taken to limit the amount of water their use can be extended to such ?elds as canning and 35 soluble material that is included in the coating. Al the like. Perhaps the most widely spread corrosion-re though as much as 5 or 6% of such materials can gen sistant treatment is the application of a tin plating. Al though the plating of tin has proven to be very satis erally be tolerated, it is preferred to use reducing agents that do not leave such materials in the ?nal product. Potassium iodide is not a preferred type of reducing agent factory in most respects, it is fairly expensive and requires in this country that the tin be imported from abroad. 40 inasmuch as it is carried through as Water-soluble potas According to the present invention, very effective corro sium compounds after reducing the chromic acid. Phos sion resistance can be imparted to members made of the phorous acid is a better reducing agent even though it is above steels, particularly where these members are to _ oxidized to phosphoric acid because the drying operation come into body-engaging contact, by applying a coating converts such phosphoric acid to water-insoluble phos of an in situ formed combination of hydrated chromium 45 phates. The most effective reducing agents appear to be oxides containing about 20 to 60% chromium by weight, the organic polyalcohols since they are readily oxidized about 40 to 95% of the chromium by weight being tri at the higher temperatures and are fairly stable at the valent, the remainder being hexavalent, and the coating lower temperatures. weighing about 10 to 200 milligrams per square foot of As indicated above, the coatings of the present inven surface that it covers. The steel is preferably subjected 50 tion are preferably applied to a steel that has had its to a grain-boundary etch, or is oxidized so that its surface surface subjected to a grain-boundary etch or covered is covered with a ?lm of uniformly adherent iron oxide, with an oxide ?lm. The etching may vary from an in before the coating is applied. signi?cant amount to a heavy etch that removes 100 to A can of lubricating oil or roasted coffee beans can 1000 milligrams of metal per square foot of steel surface. have its inner surface covered with the ‘above coating ‘to 55 With etching of more than about 400 milligrams per prevent rusting prior to ?lling as well as when ?lled with square foot, the best results are not obtained unless the oil or coffee beans; the outer surfaces of the can can also mixed chromium oxide coating weighs more than about have the same type of combined chromium oxide coating 30 milligrams per square foot. The oxidation can be which provides an excellent substrate for subsequent litho effected by merely attacking the surface with a reagent graphic coatings, varnishes, lacquers, enamels and other 60 that converts it to oxide with substantially no removal organic coatings. Instead of applying the coating of the present inven of metal. . For etching the steel surface can be contacted with tion to the can after it is manufactured or after it is ?lled aqueous nitric acid having a concentration of from 1 to and sealed, it is simpler to make the can from steel sheets 20% HNO3 by weight for a period of from 2 to 70 sec that have the coating applied while they are being pro 65 onds at a temperature of from 60 to 150° F. Aqueous duced. This permits the coated sheets to be stored, if ' solutions of ammonium persulfate, picric acid or ferric desired, and even shipped over substantial distances with nitrate are also effective and react similarly to nitric acid out requiring any supplemental treatment to prevent them solutions. For example, an aqueous solution containing from becoming unmarketable or unsightly as a result of 4% of ammonium persulfate will, when applied at 75° F . corrosion. 70 for 15 seconds, remove approximately 380 milligrams of The sheets are readily coated by ?rst making sure they metal per square foot and provide a very satisfactory sur are very clean and then passing them through an aqueous face for the chromium oxide coating. A 11/z% picric 3,031,333 3 acid solution in water at 155° F. for 12 seconds removes 4 The same coated sheet can be used for making both the top and bottom covers of a can, although in some cases different thicknesses of metal can be used in the different portions, so that separate coated sheets are re about 370 milligrams of metal per square foot and is also very satisfactory. Aqueous ferric nitrate in a concentra tion of about 5% based on anhydrous material, used at quired. 80° F. for 15 seconds removes about 400 milligrams per After the can body is secured together, the cover on square foot and also leaves an excellent pretreatment one end can be applied in any convenient manner such surface. The oxidation can be accomplished with hot as the one generally used in the industry. The can can concentrated aqueous solutions of sodium hydroxide con then be ?lled and the remaining cover applied with the taining sodium nitrate as used in standard bluing opera tions. A concentration of 39% NaOH and 2% NaNOB 10 usual precautions in the case where the contents have to be sterilized or heated, or kept in a special atmosphere used at 280° F. is very effective although any bluing treat such as under evacuation or superatmospheric pressures. ment appears to be suitable. The cans having the coatings of the present invention are It is not desirable to have oxide pretreatment ?lms that particularly useful in storing such materials as dried are loose or non-adherent; the oxide produced should be foods, nuts, spices, dough mixtures, etc., as well as the continuous and adherent to the metal surface. However, above-mentioned coffee and lubricating oil. Wet-packed such ?lms as ordinary corrosion do not interfere with the foods or other materials can also be stored in the cans application of the coating since loose portions are readily of the present invention, but here it is desirable to have removed as by a conventional pre-cleaning operation. The proportion of reducing agent to chromic acid should be insufiicient for the complete reduction of all the chromic acid to trivalent condition. The minimum amount of reducing agent is somewhat below that which will stoichiometrically reduce the lowest proportion of the chromium, inasmuch as some of the chromium is re an organic covering layer (enamel, lacquer, varnish) ap plied over the coating of the present invention on the inside surface of the can. The coatings of the present invention contribute a sub stantial amount of increased adhesion and wear resistance for such organic coatings as well as resistance to cor: duced during the drying operation even if the reducing 25 rosion. Suitable organic layers are those usually loosely referred to in the art as enamels, sanitary enamels or lac agent is not present in the bath. Chromic acid solutions quers such as the oleo-resinous phenolic or vinyl resin of any concentration can be used and the coating weight varnishes. Particularly effective forms of such organic adjusted by controlling the amount of solution that is type coating are described in U.S. Patents Nos. 2,231, left on the metal surface when it is being dried. The ?nal heat treatment is somewhat more effective if 30 407, 2,299,433, 2,479,409 and 2,675,334. Such top coat ings will even further reduce corrosion as well as increase carried out at temperatures substantially above 212 de the wear resistance and lower the contamination of the grees F. Temperatures between 250 degrees F. and 350 can contents by the oxides. degrees F. are preferable and provide the most corrosion The advantages of the present invention are contributed resistant and adherent forms of coating. At about 450 degrees F. and higher, however, the coating appears to 35 to any plain carbon steel, that is steel that contains no more than about 2% of alloying metals. They can have be adversely affected. a carbon content varying from extremely low values, In accordance with the above, cans can be made from 0.05% or even less, to as much as 1.4% or higher. The unfabricated full bright ?nished sheet steel 10 mils thick phosphorus and sulphur contents can range from sub by subjecting the sheet to the following operations: 40 stantially zero up to several tenths of a percent. Gen A. Clean the sheet cathodically in an aqueous solution erally phosphorus maxima are about 0.15% and sulphur containing 16 grams KOH per liter using a current maxima about 0.3%. These materials include the steels density of 15 amperes per square foot of cathode at ordinarily considered as carbon steels (SAE 1010 to 140—160° F. for 10 seconds. ow 1095), free cutting steels, plain carbon tool steels, in- . Cold water rinse. cluding those that have up to several percent of silicon, and casting metals. grams KOH per liter using a current density of 15 am-t The inorganic mixed chromium oxide coating of the peres per square foot of anode at l40~160° F. for 10 present invention is even further improved if ‘after the seconds. ?nal high temperature drying it is subjected to a quench . Cold water rinse. 50 that rapidly reduces its temperature at least about 25° . Flood with a passivity-preventing 1/2 % aqueous H2804 F. Any liquid appears to be suitable for this purpose, solution by weight ‘1-2 seconds at 80° F. and plain or tap water is very effective. The addition . Cold water rinse. of 0.03 to 1.0% CrO3 in the quenching Water even . Spray with an aqueous solution containing 2% nitric further improves its effectiveness. acid, 80° F., 8 seconds using a spray pressure of The mixed chromium oxide is also improved by in about 6 pounds per square inch. cluding with these oxides a small amount of an oxide of . Clean anodically in an aqueous solution containing 16 wecanHm . Cold water rinse. Desmut by brushing in water to remove loose or non-adherent reaction products including any devel oped in G. 1. Flood with an aqueous solution containing 4% chro mic acid and 1.3% cane sugar at 75° F. for 2 seconds. K. Roll through rubber rolls wetted with the ?ooded so lution. L. Cure by passing the resulting ?lmed metal through a a metal such as molybdenum that is in the same periodic group as chromium. Such supplemental oxide can be added to the chromic acid bath as water~soluble oxides, 60 acids, or as water-soluble salts. It is preferred to add molybdenum as M003. The advantages of the supple mental oxides are felt when they make up as much as 1/3 the weigh-t of the ?nal coating, although it is preferred to use from 5 to 20%. The combined oxide coatings show particularly good adhesion to thestecl with or with drying unit having a set of ceramic gas burners heated red hot by burning gas, a ?ve second exposure to the out a previous oxidation, ‘as Well as better adhesion to incandescent units being used, and the metal reaching other metals such as titanium and vanadium can also be an organic top layer. Similarly reducible compounds of a temperature of 300 to 350° F. used. In general, it is desirable to diminish the propor The addition of a wetting agent such as 0.005% of the 70 tion of reduced chromium oxide in the final coating, when a supplemental oxide is used, to not over approximately product made by condensing 3 mols of ethylene oxide 75%. with p-(n-octyl) phenol, as described in U.S. Patent No. The inclusion of 1 to 5% of insoluble or slightly solu 2,115,192, improves the Wetting in step I. ble ohromate of such divalent metals as zinc, calcium and If the metal is in the form of an elongated strip, it may 75 strontium also improves the quality of the coating in a be coiled up directly after step L. 5 similar manner. 3,031,833 This effect is obtained with or without the supplemental oxide. The improvements contributed by the quenching as described ‘above are also produced with the coatings that have the supplemental oxides, or the slightly soluble chromates, or both. The above coating techniques can be readily carried out either in a batch process or continuously. They may, for example, be added at the end of a standard 6 well as reduces friction to simplify fabrication operations such as stamping, bending, etc. Under some conditions the nitric acid treatment of the steel does not produce the desired effect unless the steel is subjected to a preliminary activation. Under these conditions the steel appears to be passive, and any chemi cal attack as by very dilute sulphuric acid or even me chanical working of the steel will su?ice to activate it. The addition of 1/2 to 2% urea to the nitric acid bath sheet steel production line. In fact, the coating treat 10 tends to suppress NO formation, thereby reducing the ment of the present invention can be carried out with the steel moving at a relatively high speed through the necessary treating stations. Under some conditions, par ticularly Where the steel is moving very rapidly through a chromic acid bath, it is desirable to have a wetting 15 tendency for iron to be oxidized to ferric form. The urea is consumed in the process and can therefore be replenished either on a continuous basis or by infrequent additions. As stated previously, etching and oxidizing with HNO3 agent present in the bath. This enables the bath liquid can be effected with concentrations of HNO3 from 1 to to more rapidly and uniformly wet the surface of the 20% and temperatures from 60 to 150° F. Not only is metal. Wetting agents of any type can be used so long it desirable from a handling and economic standpoint to as their wetting action is not completely destroyed by the work with low concentrations of HNO3 (less than 5% oxidizing action of the bath. Anionic, cationic or non 20 by weight) and low temperatures (less than 110” F.) but ionic types of wetting agent used in amounts of about extremely advantageous from an operational standpoint. 0.001 to 0.2% by weight of the bath are effective to The lower concentration and temperature provide some keep from developing coating irregularities apparently what more uniform results and greatly minimize the due to air bubbles trapped on the metal while moving tendency for oxidation of ferrous iron to ferric iron. It through the chromic acid bath. A highly elfective ex 25 should be noted that this oxidation consumes excessive ample of wetting agent is the polyoxyethylene ether of amounts of nitric acid and is accompanied by the genera alkylated phenols such as those produced by condensing tion of large quantities of noxious nitrogen oxide fumes. dodecyl phenol with twelve molecules of ethylene oxide. Other articles with which the invention is very effec Reference is also made to U.S. Patents 1,970,578 and tive are shown in U.S. Patent 2,773,623, granted Decem 2,085,706 for more speci?cally disclosed wetting agents 30 ber 11, 1956. that are suitable. The coatings of the present invention can be applied to Another example of the coating process of the present articles after they are formed as by shaping, stamping, or invention is as follows: even casting. When treating such formed articles with A. Sheets of 14 mil thick SAE 1010 steel are cleaned by the liquid coating solutions, however, care should be immersing for ?ve seconds in a 180° F. aqueous solu 35 taken to see that the layer of solution on the article does not concentrate as by running or dripping to any portion tion of 5% disodium phosphate and 3% sodium car bonate. of the article that should be protected. A localized thick . The cleaned sheets are rinsed with water at 70° F. layer of solution tends to form on the lower edges of the . Immerse the rinsed sheets in 1/10% solution of sul article under the in?uence of gravity, but a blast of air 40 can be used to redistribute the thick layer. Alternatively, furic acid in water ‘for one second at 70° F. . Rinse again in water at 70° F. the articles can be rotated or kept moving so as to keep . Subject the resulting sheets to a uniform action of jets of an aqueous solution containing 1.2%nitric acid at a thickened film from forming at any location. 80° F. for two seconds, the jets impinging at a velocity Although it is indicated above that coating weights of at least ten milligrams per square foot are required to give of ten feet per second. 45 the exceptional corrosion resistance for some purposes, . Rinse the acid treated sheets in cold water at 70° F. such as to anchor organic covering layers, coatings of . Pass the sheets between rotating brushes having stain somewhat lower weight will provide unusually good re sults. less steel bristles, to remove loose material. H. The rinsed sheets are sprayed with an aqueous solu The Weight of the coating is readily determined by dis tion of 31/2% chromic acid, 1% pentaethylene glycol 50 solving it in the flux or in a strong alkaline solution such ‘and 0.1% of the wetting agent made by condensing 2, . 3 mols of ethylene oxide with p-(n-octyl) phenol, as described in U.S. Patent No. 2,115,192. This solu tion was prepared in the mixing nozzle by supplying as 20% NaOH in water at 180° F. and weighing the metal before and after. The solution thus made can have its hex-avalent chromium content determined by titration with sodium thiosulfate. The total chromium content can be it with two separate streams, one being an aqueous 55 determined by oxidizing another portion of the solution solution of the chromic acid, and the other an aque with sodium peroxide and then again titrating with so ous solution of the remaining ingredients. I. The sheets carrying the above solution are passed dium thiosulfate. Alternatively, the coating can be scraped off, dissolved and analyzed. through an air oven, the inside of which is held at The present application is a continuation-in-part of ap 800° F., the surface of the sheets reaching a tem 60 plication Serial No. 433,698, ?led June 1, 1954, now perature of 375° F. Patent No. 2,773,623, granted December 11, 1956. That J. The hot sheets are quenched water at 70° F., application is in turn a continuation-impart of Serial No. and then permitted to dry. The dried product is extremely resistant to corrosion, 277,286, ?led March 18, 1952, now Patent No. 2,768, 103, granted October 23, 1956; Serial No. 278,481, ?led particularly if coated with an acrylate lacquer or even 65 March 25, 1952, now Patent No. 2,768,104, granted Oc a thin ?lm of methylmethacrylate resin. Other resins tober 23, 1956; and Serial No. 371,427, ?led July 30, 1953, now Patent No. 2,777,785, granted January 15, such as those made with ester-type Waxes, carnauba wax for example, are also very effective. 1957. Obviously many modi?cations and variations of the The oxide-coated steel of the present invention is fur ther protected against corrosion by applying to the coat 70 present invention are possible in the light of the above ing a ?lm of an oil such as a para?in or a glyceride oil. teachings. It is, therefore, to be understood that within Thus mineral oil or palm oil can be applied in very min the scope of the appended claims the invention may be ute quantities (0.5 milligram or more per square foot) practiced otherwise than as speci?cally described. as by conventional electrostatic coating techniques and What is claimed is: enhances the corrosion resistance of the oxide coatings as 76 1. A plain carbon steel the surface of which is etched 3,031,333 7 With ammonium persulfate, the etched surface having a corrosion-inhibiting coating of an in situ formed combi nation of hydrated chromium oxides containing about 20 to 60% chromium by weight, about 40 to 95% of the chromium by weight being trivalent, the remainder being hexavalent, and the coating weighing about 10 to 200 milligrams per square foot of surface that it covers. 2. A plain carbon steel the surface of which is picric acid etched; the etched surface being covered with a cor rosion-inhibiting coating of an in situ formed combina tion of hydrated chromium oxides containing about 20 to 60% ‘chromium by Weight, about 40 to 95% of the 8 chromium by Weight being trivalent, the remainder being hexavalent, the coating weighing about 10 to 200 milli grams per square foot of surface that it covers. References Cited in the ?le of this patent UNITED STATES PATENTS 1,801,504 1,918,661 2,741,397 2,773,623 2,797,023 Hothersall ___________ __ Phelps ______________ __ ShOttOn _____________ __ Schuster et a1 _________ __ Kaercher et al. _______ __ Apr. July Apr. Dec. June 21, 18, 10, 11, 25, 1931 1933 1956 1956 1957 Age‘.